Mobile communication technology recently trends toward rapid data service and miniaturization. Accordingly, there have been many efforts to achieve a multi system on-chip technology and develop a lower power consumption chip.
Mobile communication stations often have sleep modes to conserve energy and it is found that, in general, a typical mobile station will be in sleep mode about 92% out of total stand-by time. One conventional method for reducing power consumption during sleep mode in a modem chip of a mobile station is to control a speed of an operation clock signal of a processor built in a modem, according to an operation mode of the processor. In other words, during an idle mode when a control signal is transmitted and received between a mobile station and a base station, a main clock signal applied to a modem chip is used as a clock signal of a processor. Meanwhile, during a sleep mode when data and control signal do not need to be transmitted and received therebetween, the main clock is divided to be used as a process clock.
One example of such a conventional method is shown in FIG. 1. A main clock signal M_CLOCK of comparatively high frequency is used as a processor clock signal P_CLOCK during an idle mode. On the other hand, the main clock signal M_CLOCK is divided at a predetermined ratio during a sleep mode to form a signal of lower frequency that is then supplied to a process clock signal P_CLOCK.
By the above manner, power consumed in a processor during the sleep mode is saved insofar as the main clock signal is divided to the processor clock signal. Nevertheless, the main clock signal M_CLOCK of a high frequency remains and a special logic circuit for dividing the main clock signal is needful. Therefore, it is necessary to maintain power consumed in an input/output (I/O) pad and a divide logic circuit of a processor that receives the main clock signal M_CLOCK. For example, if the frequency of the main clock signal M_CLOCK is 30 MHz, the current consumed in the I/O pad is typically 3-4 mA. As a result, the conventional method has a limitation to reduce power consumption of a CDMA modem chip. A better way of reducing sleep mode power consumption is needed.